1. Field of the Invention
The present invention relates to a pixel circuit and an organic light emitting display device including the same, and more particularly, to a pixel circuit, a driving method thereof, and an organic light emitting display device including the same, which compensate for a threshold voltage of a driving transistor that controls emission of light from a light emitting element.
2. Discussion of the Related Art
Recently, with the advancement of multimedia, the importance of flat panel display (FPD) devices is increasing. Therefore, various types of FPD devices such as liquid crystal display (LCD) devices, plasma display panel (PDP) devices, field emission display (FED) devices, and light emitting display devices are being used. In such FPD devices, the light emitting display devices have a fast response time of 1 ms or less and low power consumption, and have no limitation in a viewing angle because the organic light emitting display devices self-emit light. Accordingly, the organic light emitting display devices are attracting much attention as next generation FPD devices.
Generally, light emitting display devices are display devices that electrically excite a light emitting material to emit light, and are categorized into inorganic light emitting display devices and organic light emitting display devices depending on a material and a structure thereof.
FIG. 1 is a circuit diagram schematically illustrating a pixel circuit of a general organic light emitting display device.
The pixel circuit of the general organic light emitting display device, as illustrated in FIG. 1, includes a switching transistor ST, a driving transistor DT, and a capacitor C, and a light emitting element OLED.
The switching transistor ST is turned on by a scan signal supplied to a scan line SL, and supplies a data voltage Vdata, supplied from a data line DL, to the driving transistor DT.
The driving transistor DT is turned on with the data voltage Vdata supplied from the switching transistor ST, and controls a data current Ioled which flows from a driving voltage Vdd terminal to the light emitting element OLED.
The capacitor C is connected between a gate and source of the driving transistor DT, stores a voltage corresponding to the data voltage Vdata supplied to the gate of the driving transistor DT, and turns on the driving transistor DT with the stored voltage.
The light emitting element OLED is electrically connected between a drain of the driving terminal DT and a ground voltage Vss terminal, and emits light with the data current Ioled supplied from the driving transistor DT. Here, the data current Ioled flowing in the light emitting element OLED is determined according to a gate-source voltage Vgs of the driving transistor DT, a threshold voltage Vth of the driving transistor DT, and the data voltage Vdata.
The pixel circuit of the general organic light emitting display device controls a level of the data current Ioled, which flows from the driving voltage Vdd terminal to the light emitting element OLED, with a switching time of the driving TFT DT based on the data voltage Vdata to emit light from the light emitting element OLED, thereby displaying a certain image.
However, in the pixel circuit of the general organic light emitting display device, the data current Ioled flowing in the light emitting element OLED may be changed due to a threshold voltage deviation of the driving transistor DT and a drop of a driving voltage Vdd. Therefore, despite the same data voltage Vdata, the data current Ioled output from each of the plurality of driving transistors DT is changed, and due to this, the pixel circuit of the general organic light emitting display device cannot realize a uniform quality of an image.
In addition, as the size of organic light emitting display devices is enlarged, the threshold voltage deviation of the driving transistor DT and the drop of the driving voltage Vdd become more severe. Due to this, an image quality of organic light emitting display devices having a large size is degraded.
That is, the light emitting element OLED is a current control element, and a current flowing through the light emitting element is controlled by the driving transistor DT connected to the light emitting element OLED. Here, due to a process differential, a threshold voltage and mobility of the driving transistor DT that controls a current are determined differently between a plurality of pixels. Therefore, even when a data signal (a data voltage) corresponding to the same gray scale is supplied to the driving transistor DT, a plurality of the light emitting elements OLED emit light having different luminance due to a threshold voltage difference and mobility difference between a plurality of the driving transistors DT. Also, due to a circuit resistor, the driving voltage Vdd applied to the light emitting element OLED is changed between when light is emitted and when light is not emitted. Due to this, the light emitting element OLED emits light having luminance different from desired luminance. That is, due to such problems, organic light emitting display devices of the related art have non-uniform luminance. As a size of organic light emitting display devices is enlarged, the above-described problems become more severe.